Essential Edition: Frequently Asked Questions about PoE Power Supply

In this issue, we will share common problems with PoE power supply.

Table of contents
What are PSE and PD?
How to understand the difference between standard PoE and non-standard PoE in one sentence?
What wire cores are needed for standard PoE power supply?
Are the Alternative A and Alternative B standards of standard PoE compatible with each other?
Can the standard PoE powered network cable use a 4-core network cable or must it be an 8-core network cable?
What are the standard PoE power supply voltage and current?
What is the difference between IEEE802.3at and IEEE802.3af? How to choose PoE products?
Standard PoE power supply, how to calculate line loss?
How to evaluate whether the power of the PoE switch meets the needs of PD?
What happens if the total power of the PD is greater than the maximum output power of the PoE switch?
Some manufacturers claim to support POE 200-meter power supply?

PoE FAQ
1. What are PSE and PD?

PSE: Power Sourcing Equipment, PoE power supply equipment, such as PoE switch;
PD: Powered Device, PoE powered device, such as wireless AP.
2. How to understand the difference between standard PoE and non-standard PoE in one sentence?
Standard PoE:


In one sentence: Power supply equipment and powered equipment negotiate through agreements to provide power supply technology with dynamic power consumption.

Passive PoE: There is no technical standard or negotiation mechanism. When supplying power, it is equivalent to connecting the tail cable of the power adapter to the network cable, and the power receiving end is equivalent to merging the power interface of the device with the network port. Each manufacturer defines it based on their specific product.
3. What wire cores are needed for standard PoE power supply?
There are two standard PoE power supply methods: Alternative A and Alternative B.

Alternative A uses 1(-), 2 (-), 3 (+), 6 (+) or 1(+), 2 (+), 3 (-), 6 (-) cores of the network cable for power supply.


Alternative B uses the 4 (+), 5 (+), 7 (-), and 8 (-) cores of the network cable for power supply.


Since PD supports both Alternative A and Alternative B, you don’t have to worry about incompatibility of power supply methods when choosing a PoE switch.

4. Are the Alternative A and Alternative B standards of standard PoE compatible with each other?
According to the protocol standards:
PSE equipment: such as PoE switches, only need to support one of the power supply methods.
PD equipment: such as AP and IPC, need to support both power supply methods.
Note: Since PD supports both Alternative A and Alternative B, you don’t have to worry about incompatibility of power supply methods when choosing a PoE switch.

5. Can the standard PoE power supply network cable be a 4-core network cable or must it be an 8-core network cable?
100M PoE switch:

Alternative A: The network cable only needs to connect cores 1, 2, 3, and 6.
Alternative B: An 8-core network cable is required and all 8 cores must be connected.
Gigabit PoE switch:

Due to the need for data transmission, both Alternative A and Alternative B require all 8-core network lines to be connected.
6. What is the standard PoE power supply voltage and current?
IEEE 802.3af: The agreement defines the voltage range 44-57V.
IEEE 802.3at: The agreement defines the voltage range 50-57V.
In standard PoE, the PSE power supply voltage is fixed, and the PD device does not need to care about the voltage provided by the PSE.

7. What is the difference between IEEE802.3at and IEEE802.3af? How to choose PoE products?

The IEEE802.3at standard is downwardly compatible with the IEEE802.3af standard. When selecting PoE power supply equipment, you only need to judge based on the power (power consumption) of the PD.
If the maximum power of the PD is less than 12.95W, you can choose either at or af products.
 If the PD maximum power is greater than 12.95W, you must choose at products.
8. For standard PoE power supply, how to calculate line loss?
The wire gauge will be marked on the network cable. Generally, Category 5 or Category 5e network cable is 24AWG (wire diameter size)


According to the AWG wire diameter comparison table, the resistance of a single wire core of a standard 100m network cable should be less than 8.94Ω.


9. How to evaluate whether the power of the PoE switch meets the needs of the PD?
Total power consumption of PoE switch > PD maximum power consumption * number of PDs + total line loss

Notice:
Some PD devices may experience instantaneous high power under some special working conditions (such as the moment when the IPC switches between day and night mode, or when some devices are started). Therefore, when purchasing a PoE switch, be sure to have redundancy for the total power of the PoE switch and reserve a certain amount of space.

10. What happens if the total power of the PD is greater than the maximum output power of the PoE switch?
The PoE interface of some PoE switches has power supply priority. When the total power of the PoE powered devices is greater than the maximum output power of the PoE switch, the PoE switch will give priority to ensure that the high-priority connection port is powered.

If the total power of the PD is greater than the maximum output power of the PoE switch, it will only cause the PD to work abnormally and will not cause damage to the switch or PD.
11. Some manufacturers claim to support POE 200-meter power supply. Can our products support it?
The standard PoE maximum transmission distance is 100m. But in fact, the main reason (bottleneck) for the 100m distance is: the maximum transmission distance of Ethernet communication is 100 meters, and the distance of PoE power supply can actually support more than 200m (the laboratory test shows that PoE power supply is no problem at 200 meters, and the power supply loss is very small).

Therefore, if power is required over longer distances:

If the quality of the network cable is good enough, as long as the network can communicate normally, there will usually be no problem with power supply. For example, for a network cable with guaranteed quality, there will be no problem for about 150 meters.
If you use a managed PoE switch, you can force the port to 10Mbps mode, which usually provides power for about 200 meters.